This invention relates to solid polymer electrolytes useful in rechargeable batteries, power supplies, capacitors and microelectrochemical sensors.
Lithium has long been regarded as a desirable component of galvanic cells. It is inexpensive and its high reduction potential (E.degree..sub.red =-3.024 v) and light weight have often suggested its use as the anodic component in high energy-density storage batteries. The reactivity of Li with water has required the use of non-aqueous electrolytes such as organic soluble lithium perchlorates, or fused lithium halides. Li.sup.+ conductors which are solid and are more highly conductive at lower temperatures than previously used electrolytes would be quite useful in such utilities; these conductors also serve to separate the anodic and cathodic components.
Solid electrolytes and in particular completely solid state galvanic cells offer special advantages as high-energy density, high-power density primary batteries. These advantages include the possibility for long shelf life, broad temperature limits of operability and miniaturization. A Li.sup.+ conducting solid electrolyte would provide the basis for a lighter and lower cost alternative to the solid state batteries which rely in the transport of Ag.sup.+ in various silver halides.
Solid electrolytes for use in batteries and other electrochemical devices must have good ionic conductivity in addition to excellent film forming properties and good storage stability. Moreover, the solid electrolyte must be simple to produce.
It is known that inorganic solid electrolytes such as Na--.beta.--Al.sub.2 O.sub.3 and Na.sub.1.sup.+.sub.x Zr.sub.2 P.sub.3-x Si.sub.x O.sub.13 (where x is 0 to 3) have good ionic conductivity. However, these inorganic solid electrolytes have very low mechanical strength and are difficult to process into a flexible film.
Further, it is known that complexes of certain polymeric materials and various salts of metals belonging to Group I or Group II of the Periodic Table (e.g., LiCF.sub.3 SO.sub.3, LiI, LiB, LiClO.sub.4, NaI, NaCF.sub.3 SO.sub.3 and KCF.sub.3 SO.sub.3) function as solid electrolytes. These polymers include polyethylene oxide (PEO), polypropylene oxide (PPO), poly(ethylene adipate) (PEA), poly(ethylene succinate) (PES), polyphosphazine (PPhz), polysiloxane, poly(N-methylaziridine) (PmAZ), triol type PEO crosslinked with dysfunctional urethane, PEO-PPO-PEO block copolymer crosslinked with trifunctional urethane, and the like. These complexes have good pliability and viscoelasticity, both of which are inherent to polymeric materials, and are easy to process.
High energy density, rechargeable solid polymer electrolyte-using solid state batteries promise virtually maintenance-free reliable operation over many thousands of cycles. However, the ionic conductivity of the above polymers is highly temperature dependent. Although the ionic conductivity is good at temperatures above room temperature, the ionic conductivity abruptly decreases at room temperature or lower. Thus it is difficult to use these polymers in general purpose commercial products for use over a wide temperature range.
Accordingly, it is an object of the present invention to provide a method for increasing the ionic conductivity of solid polymer electrolytes, particularly at and below room temperature.
It is another object of the present invention to provide a solid state battery system exhibiting reliable operation at and below room temperature.
Other objects and advantages of the invention will be apparent to those skilled in the art.